The invention relates to a drop-catch mechanism for preventing a door leaf of an overhead door from falling down upon breakage of a balancing spring thereof, the mechanism comprising an input member for coupling with a balancing spring of the overhead door, a ratchet wheel for coupling with a winding axle carrying the door leaf of the overhead door, a pawl, and means for moving the pawl from a free position that allows rotation of the ratchet wheel to a catching position that blocks rotation of the ratchet wheel, the means for moving the pawl being configured such that, in use, breakage of a balancing spring coupled to the input member, causes the pawl to move from the free position to the catching position.
Such a drop-catch mechanism is generally known and is used to block rotation of the winding axle of an overhead door to which a singular or articulated door leaf is directly or indirectly coupled, due to a resulting gravitational force on the door leaf upon breakage of a balancing spring.
In use, the input member of the drop-catch mechanism is coupled to a first, stationary end of a pretensioned torsional balancing spring that has a second, rotary end coupled to the winding axle of the overhead door. The ratchet wheel is directly or indirectly coupled to the winding axle, such that it can cooperate with the pawl of the drop-catch mechanism to block rotation of the winding axle.
In the known drop-catch mechanism, the means for moving the pawl between the free position and the blocking position comprise a catch member that is displacably guided between a first position, corresponding to the free position of the pawl, and a second position corresponding to the catching position of the pawl. In use, the balancing spring biasses the catch member into the first position. Upon breakage of the balancing spring, the catch member moves into the second position.
The pawl comprises a lever that extends between a first end that is pivotably carried on the base member and a second end that is laid-up onto the catch member. When the catch member moves from the first position into the second position, it moves away from the second end of the pawl and the pawl is free to move into the catching position under the action of a pawl spring.
A problem associated with the known device is that movement of the catch member from the first into the second position does not by itself ensure movement from the pawl from the free position to the catching position. The catch and the pawl comprise two sequentially disposed drives of which the movement is indirectly coupled. In particular, after the catching member has moved from the first into the second position, the pawl may remain in the free position, e.g. when the pawl spring malfunctions or when the force exerted by the spring is not great enough to overcome friction in the pivot of the pawl, e.g. due to corrosion or built-up dust. Furthermore, the laid-up lever arm of the pawl requires relatively much space, which makes the drop-catch mechanism difficult to use in domestic overhead garage doors, as in a domestic garage normally only a small amount of space is available for the drop-catch mechanism. A typical domestic overhead garage door has e.g. two helically wound torsion springs having a diameter of 44.5 mm (1¾ inch), 51 mm (2 inch) or 67 mm (2⅝ inch), a tubular winding axle having a diameter of approximately 25.4 mm (1 inch) and a maximum door leaf weight of 87 kg per spring. For domestic overhead garage doors, the reliability of the drop-catch mechanism is especially important, as these doors are in practice not always subject to the prescribed regular safety inspection.